Plasma immersion ion implantation is performed by generating a plasma containing ions of species to be implanted in a semiconductor wafer or workpiece. The plasma may be generated using a plasma source such as a toroidal plasma source at the reactor chamber ceiling. Ion energy sufficient to achieve a desired ion implantation depth profile below the wafer surface is provided by coupling a very high RF bias voltage (e.g., 10 kV to 20 kV) to the semiconductor wafer through an insulated cathode electrode within the wafer support pedestal. Such a high bias voltage requires a correspondingly high D.C. electrostatic wafer clamping voltage on the electrostatic chuck to hold the wafer during plasma immersion ion implantation. High implant dose rate requires a high plasma ion density, which is achieved using a toroidal plasma source operating at a low chamber pressure. The requisite ion implant depth profile requires a very high ion energy, which is achieved by applying a very high RF bias voltage across the plasma sheath at the wafer surface. The process gas employed in plasma immersion ion implantation can be a fluoride or a hydride of the dopant species to be implanted.
In DRAM/flash memory fabrication, it is necessary to implant a semiconductor dopant species into the polycrystalline silicon (polysilicon) gate electrodes to increase their conductivity. The gate electrodes are formed by depositing amorphous silicon on a thin gate oxide layer and then annealing the wafer sufficiently to transform the deposited silicon from the amorphous state to a polycrystalline state. The polycrystalline silicon gate layer thus formed is about 20 nm to 80 nm thick. The implanted species is one that promotes p-type semiconductivity in silicon, such as boron, or n-type semiconductivity, such as arsenic, phosphorus, and antimony. The plasma immersion ion implantation process must be carried out for a sufficient time to attain a required ion implant dosage in the polysilicon gate layer, corresponding to an electrical resistivity in the range 100-1000 Ohm/sq.
The polysilicon gate electrode must be kept free of metal (e.g., aluminum) contamination. Such contamination is caused by sputtering of the chamber interior surfaces during the plasma immersion ion implantation process, which introduces metal atoms into the plasma environment facing the gate electrode.